As a method of forming, at the center of a plate-like metallic blank, a case-like boss projecting from one lateral side of the blank, there are conventionally available a cold forging method, a drawing method, a burring method and the like.
Of these conventional methods, the cold forging method is adapted to form a case-like boss with a plastic flow of material itself utilized. Accordingly, when there is formed, from a thick plate-like metallic blank, a pulley such as a crank shaft pulley requiring a boss having a small inner diameter and a relatively great projecting height, it is difficult to form such a boss having a predetermined projecting height, even though a large-size press machine of the class of 2,000 tons to 2,500 tons is used.
In the drawing method, material flows due to drawing, so that the resulting product and particularly the boss are excessively reduced in thickness, thus failing to provide a predetermined strength.
In the burring method, there is obtained a boss having a height substantially proportional to the diameter of a hole (which corresponds to the inner diameter of the boss) to be previously punched by a punch or the like. Accordingly, there cannot be formed a boss the inner diameter of which is small and the height of which is great. Thus, the range in which a boss can be formed, is automatically limited by the relationship between the inner diameter and height thereof.
In view of the foregoing, the present invention is proposed with the objects of providing (i) a method of forming a boss on a plate-like metallic blank, capable of forming, with high precision, a boss having a desired inner diameter, a desired thickness and a desired height with the use of a small press machine without the thickness of the initial material reduced so much in the course of the production steps, and (ii) a method of forming a pulley from a metallic plate, using the first-mentioned method.